1. Field of the Invention
The present invention relates to a clay extruder which is preferably used in manufacturing a columnar or a rectangular-columnar article including, for example, a ceramic raw material or the like. More particularly, it relates to a clay extruder capable of preventing the unevenness of a clay density caused by the rotation of a screw in the clay extruder to obtain a satisfactory a columnar or a rectangular-columnar article having high homogeneity.
2. Description of the Related Art
Heretofore, there have been known various extruders, which are used for kneading and homogenizing a clay including, for example, a ceramic material by the rotation of a screw in a drum to prepare a columnar or a rectangular-columnar article (e.g., see JP-A-9-94818 and JP-A-10-100131).
FIG. 2 is a schematic constitution diagram showing one example of a conventional clay extruder. In the conventional clay extruder, as shown in FIG. 2, a drum 2 has a structure in which a screw 34 is provided. A large shearing force is imparted to a clay fed from a supply port 19 into the drum 2 by the screw 34 in the drum 2, to knead and homogenize the clay. Then, the clay kneaded and homogenized in the drum 2 is extruded from an extrusion port 20 to manufacture a kneaded clay columnar article 31.
Moreover, as a manufacturing method of a ceramic honeycomb formed article, there is known a method of blending a ceramic material, water, a binder and the like; then preparing a large muddy lump as the clay by a kneader; degassing, kneading and homogenizing the clay by use of a clay extruder; preparing the columnar article by extrusion; and manufacturing a ceramic honeycomb formed article from this a columnar or a rectangular-columnar article by extrusion forming (a plunger type forming machine).
In this case, the columnar article is prepared by the clay extruder, but the density of the clay partially becomes uneven by the rotation of the screw of the clay extruder. Moreover, when a ceramic honeycomb structure is formed by extrusion using the columnar article, various formed article defects are generated.
Especially in recent years, the thinning of partition walls in the ceramic honeycomb structure has farther progressed. To prepare the ceramic columnar article for use in the formed article having such thin walls, homogeneity is remarkably intensely demanded, and the manufacturing steps of the columnar article need to be contrived to satisfy strict quality control requirements.
When the preparation of the ceramic honeycomb structure by the extrusion forming is performed by using the columnar article obtained by the conventional clay extruder having the structure shown in FIG. 2, unevenness is generated in the clay density of the columnar article by the rotation of the screw in the clay extruder, and the forming defect of the honeycomb structure is sometimes generated.
FIG. 3 is a schematic explanatory view showing a tip structure of the screw 34 of a conventional clay extruder 30 shown in FIG. 2. FIG. 4 is a schematic explanatory view showing the density distribution of a clay in a part shown by a broken line part A of FIG. 3. In FIG. 4, a dark part indicates a highly dense state, and a bright part indicates a lowly dense state. The abscissa indicates a rotary shaft direction of the screw 34 (an extruding direction 41), and the ordinate indicates a drum diametric direction 42. A hatched region shown in FIG. 4 shows a section of the screw 34. As shown in FIG. 4, in an abscissa direction, a portion of the clay which directly receives a thrust force generated by the rotation of a rotary blade 35, that is, the clay which comes in contact with the face of the rotary blade 35 in the extrusion direction 41 is highly densified (a broken line part B). In an ordinate direction, when the clay moves following the rotation of the screw in the drum 2, a force is applied to the centrifugal direction of the drum diametric direction 42, and a frictional force between the clay and a drum inner wall 43 is generated, so that the clay which comes in contact with the drum inner wall 43 is highly densified. Moreover, the density of the clay lowers in the periphery of a rotary shaft 36 and near the face of the rotary blade 35 opposite to the extrusion direction 41 (a broken line part C).
Therefore, when the extrusion forming and the subsequent firing of the ceramic honeycomb structure are performed by using the columnar or columnar rectangular clay article obtained by the conventional clay extruder 30 as shown in FIGS. 2 and 3, the density in the columnar or columnar rectangular clay article is uneven for the above reason, and hence various formed article defects or firing defects might be generated.
For example, FIG. 5 is a schematic sectional view from the side surface of the columnar or columnar rectangular clay article 31 obtained by using the clay extruder 30 including the conventional screw tip structure of FIG. 3. As shown in this drawing, flow patterns 17 rotationally symmetrical with respect to a central line 16 of the columnar or columnar rectangular clay article 31 and curved in the extrusion direction 41 are formed owing to the density difference inside the clay.
FIG. 6 is an enlarged schematic sectional view of the flew pattern in a broken line part D of FIG. 5 near the central line 16 of the columnar or columnar rectangular clay article 31. In a structure shown in the drawing, a highly dense clay portion and a lowly dense clay portion are alternately laminated before and after the curved flow pattern 17 with respect to the central line 16.